Recently, people have increasingly directed their attention to the pollution of environment or the exhaustion of natural resources, and to the development of measures to cope with those problems. As one such countermeasure, the development of fuel cells have been actively pursued. Among such fuel cells, there are phosphoric acid fuel cells (PAFC) and polymer electrolyte fuel cells (PEFC). Utilizing hydrogen as fuel, those fuel cells require a conversion system capable of converting hydrocarbon or methanol which serves as a material into hydrogen, and the development of an effective conversion system has remained a particularly important technical challenge in the development efforts of those fuel cells.
Materials upon which studies have been performed to convert them into hydrogen to drive PEFCs for vehicles include, for example, methanol, dimethyl ether (DME), ethanol, natural gas, propane, gasoline, etc. Among them, the most notable advancement is observed in the conversion of methanol into hydrogen, because methanol requires the lowest temperature for its conversion. Currently, three methods have been proposed for methanol conversion: water vapor-based conversion, partial oxidization-based conversion, and combination of the two methods. See Watanabe, M., “Development of PEFC and its commercialization,” pp. 141-166, May 28, 1999, Assoc. Technol. Information.
The water vapor-based conversion can be represented by the following reaction formula:CH3OH+H2O→CO2+3H2 
This is an endothermic reaction occurring at 200 to 300° C.
The partial oxidization-based conversion can be represented, when air is used as oxidizing gas, by the following reaction formula:CH3OH+1/2O2+2N2→CO2+2H2+2N2 This is an exothermic reaction occurring at 200 to 600° C.
The combinational conversion (representative example) can be represented, when air is used as oxidizing gas, by the following reaction formula:CH3OH+1/3O2+4/3N2+1/3H2O→CO2+7/3H2+4/3N2 This is an exothermic reaction where heat is generated about one third what is generated during the partial oxidization-based conversion. The reaction occurs at 400 to 600° C.
As an alternative to the above method, an invention provides a hydrogen generating system for generating hydrogen highly efficiently utilizing, as a material, hydrocarbon fuel such as natural gas, LPG, gasoline, naphtha, kerosene, etc., and water in order to provide resulting hydrogen, for example, to a hydrogen exploiting device such as a fuel cell (see Japanese Patent Publication No. 3473900). According to the invention, the system “comprises, at least, a hydrocarbon fuel supply portion, a combustion portion, a water supply portion, a gas mixing portion where fuel and water or water vapor are mixed to produce a mixed gas to be converted, and a conversion portion filled with a conversion catalyst, and is characterized in that the gas to be converted is converted, under the catalytic action of the conversion catalyst, into gas containing hydrogen, and that combustion gas waste generated by the combustion portion is used to directly heat, only through partition walls, at least the gas mixing portion and the conversion portion.” According to this system, the conversion temperature is high, that is, about 700° C. (See claim 1, and paragraphs [0001], [0017] and [0022] of the cited patent document).
As seen from the two illustrative methods presented above, for generating hydrogen conversion must occur at a high temperature not lower than 200° C., and, in addition, those conventional methods have a number of additional problems: intoxication of the conversion catalyst, admixture of CO with the conversion gas (hydrogen-containing gas) which must be removed, and admixture of air with the conversion gas which is generated by partial oxidization or by the combinational method.
On the other hand, technique has been known whereby one can obtain hydrogen-containing gas by decomposing fuel comprising an organic compound at a low temperature, and one such technique is represented by a method for generating hydrogen electrochemically and a system based on the method. A fuel cell utilizing hydrogen generated by such an electrochemical method is also known. (See Japanese Patent Publications Nos. 3328993 and 3360349, U.S. Pat. Nos. 6,299,744, 6,368,492, 6,432,284, and 6,533,919, and United States Patent Application No. 2003/0226763, and Japanese Unexamined Patent Application Publication No. 2001-297779). Japanese Patent Publication No. 3360349 cited above describes (claim 1), “a method for generating hydrogen comprising providing a pair of electrodes on the two opposite surfaces of a cation exchange membrane, contacting a fuel containing at least methanol and water with one electrode having a catalyst, applying a voltage between the pair of electrodes so that electrons are withdrawn from the electrodes thereby causing a reaction to occur on the electrodes whereby hydrogen ions are generated from methanol and water, and allowing hydrogen ions to be converted on the other electrode, being supplied with electrons, into hydrogen molecules.” The same patent document discloses another method (paragraphs [0033] to [0038]) for selectively generating hydrogen using a conversion system, the method comprising supplying water or water vapor together with methanol which serves as a fuel, applying a voltage via an external circuit to cause electrons to be withdrawn from a fuel electrode, so that reaction represented by CH3OH+2H2O→CO2+6e−+6H+ occurs on the fuel electrode, and allowing hydrogen ions thus produced to pass through a cation exchange membrane to reach the opposite electrode where the hydrogen ions undergo reaction represented by 6H++6e−→3H2. Japanese Patent Publication No. 3360349 cited above describes (paragraphs [0052] to [0056]) a fuel cell which utilizes hydrogen generated by a method as described above.
According to the inventions described in Japanese Patent Publications Nos. 3,328,993 (paragraph [0042]) and 3,360,349 (paragraph [0080]) cited above, it is possible to generate hydrogen at a low temperature. However, the methods described in those inventions are obviously different from the hydrogen generating method of the present invention and hydrogen generating system of the present invention based on the method which will be given below in following points: those methods require the application of voltage, and hydrogen is generated on the electrode opposite to the electrode (fuel electrode) to which fuel is supplied, and no oxidizing agent is supplied to the opposite electrode.
This holds true also for the inventions disclosed by U.S. Pat. No. 6,368,492 cited above similarly to Japanese Patent Publications Nos. 3,328,993 and 3,360,349 cited above. Those inventions use a system for generating hydrogen where protons generated on anode 112 serving as fuel electrode pass through partition membrane 110 to reach cathode 114 opposite to the anode, and according to the system, voltage from DC power source 120 is applied between anode (fuel electrode) and cathode (opposite electrode) to decompose organic fuel such as methanol or the like electrochemically. In addition, hydrogen is generated on the electrode opposite to the fuel electrode, and no oxidizing agent is supplied to the opposite electrode.
Japanese Unexamined Patent Application Publication No. 2001-297779 cited above discloses a fuel cell system incorporating a hydrogen generating unit. According to the disclosure (claim 1) of the invention, “Liquid fuel containing alcohol and water is supplied to porous electrode 1 (fuel electrode), air is supplied to gas diffusion electrode 2 (oxidizing agent-applied electrode) opposite to electrode 1, and a load is inserted between a terminal leading to porous electrode 1 and another terminal leading to gas diffusion electrode 2 to achieve electric connection allowing a positive voltage to be applied to porous electrode 1 via the load from gas diffusion electrode 2 which corresponds to the positive electrode of MEA2 capable of acting as a conventional fuel cell.” The same patent document further adds (paragraph [0007]), “As a result, alcohol reacts with water to produce carbon dioxide gas and hydrogen ion, the hydrogen ion passes through an electrolyte membrane 5 to reach a gas diffusion electrode 6 located centrally where the hydrogen ion is converted into hydrogen gas. On the opposite surface of gas diffusion electrode 6 in contact with another electrolyte layer 7, there arises another electrode reaction where hydrogen gas is reconverted into hydrogen ion, and hydrogen ions migrate through electrolyte layer 7 to reach another gas diffusion electrode 2 where hydrogen ions react with oxygen in air to produce water.” Thus, with this system, electric energy generated by a fuel cell is utilized to generate hydrogen on the hydrogen generating electrode (gas diffusion electrode 6) which is then supplied to the fuel cell. Moreover, the system is the same with those described in the patent documents cited above in that hydrogen is generated on the electrode opposite to the fuel electrode.
There are some other known methods for generating hydrogen (Japanese Unexamined Patent Application Publications Nos. 6-73582 (claims 1 to 3, paragraph [0050]) and 6-73583 (claims 1 and 8, paragraphs [0006] and [0019]). According to the inventions, a reaction system with a partition membrane is used where anode (electrode A) and cathode (electrode B) are placed opposite to each other with a proton conducting membrane (ion conductor) inserted therebetween, and where alcohol (methanol) is oxidized with or without concomitant application of voltage, or with concomitant uptake of electric energy. All those methods, however, are based on a method whereby alcohol is oxidized by means of an electrochemical cell (the reaction product includes carbonic diester, formalin, methyl formate, dimethoxymethane, etc.), and not on a method whereby alcohol is converted by reduction into hydrogen.”